Composition and a method for defect reduction

ABSTRACT

A metal layer cleaning composition including a first component that is an acid; and a different second component that is a chelating agent, wherein the composition is in a form suitable for use in effectively removing undesirable constituents from a wafer surface following a polishing operation.

RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 10/429,989, filed May 6, 2003, which is adivisional of U.S. patent application Ser. No. 09/476,977, filed Dec.31, 1999, issued as U.S. Pat. No. 6,592,433.

BACKGROUND

1. Field of the Invention

The invention relates generally to removal of particles from a substrateand more particularly to removal of at least one particle from a metallayer deposited on a substrate using an acid.

2. Description of Related Art

Integrated circuits are made up of literally millions of active devicesformed in or on a silicon substrate or well. The active devices that areinitially isolated from one another are later connected together to formfunctional circuits and components. The devices are interconnectedtogether through the use of well known multilevel interconnections. Across-sectional illustration of a typical multilevel interconnectionstructure 10 is shown in FIG. 1. Interconnection structures normallyhave a first layer of metallization, an interconnection layer 12(typically aluminum alloys with up to 3% copper), a second level ofmetallization 14, and sometimes a third or even fourth level ofmetallization. Interlevel dielectrics 16 (ILDs), such as doped andundoped silicon dioxide (SiO₂), are used to electrically isolate thedifferent levels of metallization in silicon substrate or well 18. Theelectrical connections between different interconnection levels are madethrough the use of metallized vias 11 formed in ILD 16. In a similarmanner, metal contacts 22 are used to form electrical connectionsbetween interconnection levels and devices formed in well 18. The metalvias 11 and contacts 22, hereinafter being collectively referred to as“vias” or “plugs”, are generally filled with tungsten 14 and generallyemploy an adhesion layer 16 such as TiN. Adhesion layer 16 acts as anadhesion layer for the tungsten metal layer 14 which is known to adherepoorly to SiO₂. At the contact level, the adhesion layer acts as adiffusion barrier to prevent W and Si from reacting.

In one process, metallized vias or contacts are formed by a blankettungsten deposition and a chemical mechanical polish (CMP) process. In atypical process, via holes 23 are etched through an ILD 24 tointerconnection lines or a semiconductor substrate 26 formed below.Next, a thin adhesion layer 28, such as TiN, is generally formed overILD 24 and into via hole 23, as shown in FIG. 2B. Next, a conformaltungsten film 29 is blanket deposited over the adhesion layer and intothe via hole 23. The deposition is continued until the via hole 23 iscompletely filled with tungsten. Next, the metal films formed on the topsurface of ILD 24 are removed by CMP, thereby forming metal vias orplugs 28.

In a typical CMP process as shown in FIG. 2C, the substrate or wafer 30is placed face-down on a polishing pad 32 which is fixedly attached to arotatable platen 34. In this way, the thin film of a metal layer to bepolished (i.e., tungsten film 29) is placed in direct contact with pad32. A carrier 36 is used to apply a downward pressure F₁ against thebackside of substrate 30. During the polishing process, pad 32 andplaten 34 are rotated while a downward force is placed on substrate 30by carrier 36. An abrasive and chemically reactive solution, commonlyreferred to as “slurry” 35 is introduced onto pad 32 during polishing.Slurries generally include an abrasive material such as alumina orsilica. The slurry initiates the polishing process by chemicallyreacting with the film being polished. The polishing process isfacilitated by the rotational movement of pad 32 relative to wafer 30 asslurry is provided to the wafer/pad interface. Polishing is continued inthis manner until all of the film on the insulator is removed.

After the polishing process, the substrate is then rinsed with asolution such as deionized water. By rinsing the substrate, particlesfrom the slurry are removed from the metallized layer.

Conventional rinsing methods include using a double sided scrubber usingdeionized water for removing particles present from the CMP from ametallized layer. However, using deionized water generally does notremove all of the particles. Another conventional method is a“magasonic” bath which involves high frequency vibration in whichparticles are shaken off the substrate. This method also leavesparticles on the metal layer.

Removing foreign particles from a substrate that is used in integratedcircuits is known in the art. One known method involves introducing aslurry over a substrate and polishing the substrate. The substrate isthen rinsed with deionized water. A scrubber then cleans the substrate.However, this method is problematic because it is unable to remove theparticles to a nondetectable level. FIG. 8 shows that a substrate usinga conventional method such as that which is described above leaves alarge quantity of defects on the substrate. Particles on a substrate mayaffect the electrical conductivity between the various layers of theinterconnect within an integrated circuit and cause catastrophicfailures upon further processing. Accordingly, it is desirable to have amethod and an apparatus wherein particles are removed from a substrateto a nondetectable level of particles without affecting adhesion or theintegrity of the post-polish metal layer surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration showing a portion of a standardmultilevel integrated circuit.

FIG. 2A is a cross-sectional illustration showing the formation of a viahole through an insulating layer formed on a conductive layer of asemiconductor substrate.

FIG. 2B is a cross-sectional illustration showing the formation of anadhesion layer and a tungsten layer on the substrate of FIG. 2A.

FIG. 2C is a cross-sectional illustration of a chemical mechanicalpolishing apparatus used to polish the films formed on the substrate ofFIG. 2B.

FIG. 3 illustrates an assembly wherein a barrier layer is formed over anoxide layer of a substrate.

FIG. 4 illustrates the assembly of FIG. 3 after the assembly hasundergone a first polishing operation.

FIG. 5 illustrates the assembly of FIG. 4 after the assembly hasundergone a second polishing operation.

FIG. 6 illustrates the assembly of FIG. 5 after it has been rinsed.

FIG. 7 provides a graphic comparison of defect density on a wafer usinga conventional method compared to one embodiment of the invention.

FIG. 8 provides a graphic comparison of defect events on a wafer using aconventional method compared to one embodiment of the invention.

FIG. 9 illustrates a schematic view of the process that may be used inaccordance with an embodiment of the invention.

FIG. 10 illustrates a polisher that may be used in one embodiment of theinvention.

FIG. 11 illustrates a scrubber that may be used in one embodiment of theinvention.

FIGS. 12A and 12B illustrate a flow diagram in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A method and a composition is disclosed for reducing defects in anelectronic device such as in an integrated circuit by using a bufferedsolution comprising an acid. The following detailed description and theaccompanying drawings are provided for the purpose of describing andillustrating presently preferred embodiments of the invention only, andare not intended to limit the scope of the invention.

One embodiment of the invention relates to a method of removing at leastone particle by polishing a metal layer over a substrate using a slurryand introducing a solution that includes an acid. Another embodiment ofthe invention relates to the same process described above except thesolution comprises an acid and a chelating agent. The solution may alsobe buffered.

Another embodiment of the invention relates to a method of using a firstsolution that is deposited onto the substrate and a polisher that has anabrasive material at the surface of a polishing pad of the polisher thatcontacts the substrate and removing the particle from the substrateusing a second solution containing an acid.

In another embodiment of the invention, a chelating agent is used in thesolution. In yet another embodiment of the invention, the solution isbuffered to allow the pH to remain constant.

FIGS. 3 through 7 show one embodiment of a fabrication process (e.g., anintegrated circuit fabrication process) incorporating the method of theinvention wherein a solution containing an acid is used to removeparticles from the surface of a substrate. FIG. 3 shows wafer 38 whereinoxide layer 40 is disposed over a substrate. Adhesion layer 42comprising of, for example, tantalum is formed over oxide layer 40.Metal layer 44 comprising an adhesion material such as copper, or anyother suitable conductive material is blanket deposited over adhesionlayer 42 and has formed a via or contact. Preferably, copper is used.FIG. 4 shows the substrate of FIG. 3 after the metal layer has beenpolished using a conventional polishing technique. One conventionalpolishing technique for polishing a metal layer of copper involvesintroducing a slurry (also referred to herein as a first agent) ofalumina or silica onto a polishing pad and the polishing pad thencontacts the metal layer. As illustrated in FIG. 4, fabricationtechniques such as chemical mechanical polishing (“CMP”) generateundesired particles on the surface of the substrate. Particles 50 thatmay have been generated from the first polishing operation or from someother source remain on barrier layer 42 and metal layer 44. Suchparticles can disrupt the integrity of electrical signals utilizing thevia and metal layer.

FIG. 5 shows wafer 38 after a second polishing operation is performed inwhich barrier layer 42 is polished such that barrier layer 42 has beensubstantially removed from the surface of oxide layer 46. CMP with anabrasive material such as silica is used to perform the second polishingoperation. The rate of removal of barrier layer 42 is approximately inthe range of 100 Å/minute to 1000 Å/minute. Even after polishing barrierlayer 42, particles 50 remain on the surface of oxide layer 46 and themetal layer 44.

In order to remove undesired particles 50 after both polishing stepsdiscussed above, an aspect of the invention includes introducing asolution (also referred to herein as a second agent) comprising an acidover the substrate. FIG. 6 shows that particles 50 from the device shownin FIG. 5 have been removed from oxide layer 46 and metal layer 44 whena solution comprising an acid is used to rinse the device of FIG. 5.Preferably organic acids are used such as carboxylic acids includingacetic acid, citric acid, gluconic acid, glucoronic acid, oxalic acid,and tartaric acid. It is to be appreciated that the list of suitableorganic acids is not exhaustive and that other organic acids may be usedparticularly those having such as multivalent carboxylic acids similarto those listed. The concentration of acid to be used depends upon theacid selected. Preferably, citric acid is used at a concentration of 50mM.

Inorganic acids may also be used such as sulfuric acid, nitric acid, andphosphoric acid. However, these inorganic acids generally must besubstantially diluted to reduce their corrosivity to prevent the surfaceof the metal layer from becoming too rough. A sulfuric acid having aconcentration on the order of less than five percent acid is an exampleof such a dilution. A rougher metal layer surface may affect adhesion ofsubsequent layers and the electrical conduction of the fabricateddevice.

In one embodiment of the invention, the solution comprising the acid isbuffered and comprises an organic acid and a chelating agent. Examplesof chelating agents include aliphatic amines, hydroxy alkyl amines,aminocarboxylic acids, cyanides, organosulphides, ammoniaethylyenediaminetetraacetic acid (EDTA), ethlyenediamine (EN),nitrilotriacetic acid (NTA), glycin, diethlyene triamine, and triethanolamine. It is generally believed that chelating agents form bonds withmetals atoms of the metal layer. It will be appreciated that otherchelating agents may be used provided that the chelating agent iscapable of forming a bond with a metal that is used in the metal layer.In the case of a copper metal layer, a chelating agent is added to bindfree (dissolved) copper ions in solution and to prevent the copper ionsfrom adsorbing on the surface of the substrate. The composition of thissolution substantially reduces particles on the surface of a substrate.

In one embodiment, the solution comprises, for example, 50 mM citricacid and 20 mM potassium citrate (or ammonium citrate as an alternativeto potassium citrate), and 100 ppm of EDTA. The solution is diluted withdeionized water. Suitable pH ranges are from 3 to 4 but the pH may rangefrom 3 to 5. The pH range from 3 to 4 increases the copper solubilityand the copper adsorption of ions on SiO₂ is minimized. This solutionprovides approximately in the range of two to one hundred timesimprovement in frontside defects. This solution also reduces backsidemetal levels such as copper to less than 10⁻¹² atoms/cm² therebyeliminating the need to have a dedicated copper processing equipment. Itshould be noted that although one embodiment of the invention describesa metallized layer being polished prior to the rinsing solution beingintroduced onto the metallized layer, the metallized layer may becleaned by introducing the rinsing solution onto the metallized layerwithout first polishing the metallized layer.

Table 1 compares the effectiveness of embodiments of the invention whena chelating agent is used and is not used in the process. As shown inTable 1, total reflection x-ray fluorescence (“TXRF”) is one method usedfor determining small amounts of copper that are readsorbed onto asubstrate. When citric acid scrub is used without a chelating agent, theaverage TXRF is 4×10¹⁰ to atoms/cm². In comparison, when a chelatingagent is combined with citric acid, the TXRF is nondetectable at adetection limit of 10¹⁰ atoms/cm². Conventional methods (i.e., a processusing a double sided scrubber and deionized water) and control with nopolish provide 9×10¹⁰ atoms/cm² and 289×10¹⁰ atoms/cm², respectively.TABLE 1 Comparison of Copper Levels on the Backside of a SubstrateHaving Copper Contamination TXRF Process Used to Clean x 10¹⁰ Average ofa Substrate Coordinate atoms/cm² TXRF Data Citric Acid Scrub (0, 0) nd 4 (0, 40) nd (63.6, 63.6) 4.4 (−63.6, −63.6) nd Citric Acid Scrub (0, 0)nd nd Wherein the Citric  (0, 40) nd Acid is Combined (63.6, 63.6) ndWith EDTA (−63.6, −63.6) nd Conventional Method (0, 0) nd 9 Using  (0,40) nd Deionized Water (63.6, 63.6) 5.6 (−63.6, −63.6) 13 ControlProcess (0, 0) 52 289 Wherein  (0, 40) 75 No Polish Is Used (63.6, 63.6)313 (Post-electroplate) (−63.6, −63.6) 716

FIGS. 7 and 8 compare the amount of defects between the conventionalprocess of using deionized water compared to a buffered citric acidsolution. For example, FIG. 7 shows that POR leaves 0.8 defects/cm²compared to approximately 0.06 defects/cm² of the buffered organic acid.FIG. 8 further shows the defects between a POR clean which is generallygreater than 140 defects on a wafer compared to the much lower defectsof about 40 defects or lower for a buffered citric acid process.

FIG. 9 shows one embodiment of the invention wherein a substrate isadvanced to a polishing operation 70 in which a slurry is put onto thepolishing pad and the polishing pad contacts the substrate and is usedduring the polishing operation. Chemical mechanical polishing (CMP) maybe used in this process. However, it will be appreciated that othersuitable methods such as orbital polishing may be used to practice theinvention. In CMP, a portion of a metal layer comprising a metal such ascopper may be removed from the substrate using a slurry that isdispensed or deposited onto the metal layer. The slurry includes anabrasive material such as aluminum, silica, or other suitable material.

A polisher 100 shown in FIG. 10 generally comprises a carrier 140 andthree platens (130, second and third platens are not shown). Each platenhas a polish pad 120. Two of the three platens may be used to remove aportion of the copper using a slurry. The slurry may contain an abrasivematerial such as alumina, silica, etc. Each platen further has fourpolishing heads 170. The polishing heads rotate on a turret.

The substrate is polished in series such that platen one 140 removes aportion of a metallized layer such as copper by polishing for a certainamount of time such as less than two minutes and the substrate is thenmoved to the second platen where another portion of the copper isremoved using a slurry. The substrate then moves to a third platen wherea portion of the tantalum is removed. At the third platen 130, thepressure is lowered to approximately 2 psi. The polish time of thisoperation is approximately 125 seconds or less.

The substrate is then advanced to scrubber 30 wherein a solution isintroduced through the polisher and is deposited over the substrate andthe substrate is scrubbed. The solution that is introduced onto thesubstrate contains an acid and preferably a chelating agent. Thesolution may also be buffered. Finally, the substrate undergoes aspin/rinse/dry cycle until it reaches a dried state. The substrate isdried for approximately 2 minutes or less at a temperature ofapproximately 22° C.

Scrubber 90 shown in FIG. 11 may be a double-sided scrubber that hasfour submodules (300, 310, 320, 330). In the first submodule 300,deionized water is sprayed onto the substrate. The substrate is thenadvanced to a second submodule 310 wherein a buffered solutioncomprising an acid and a chelating agent is applied to the substrate.The solution may be fed to the substrate through a chemical dispense arm340 onto a brush 350. The substrate is then advanced to a thirdsubmodule 320 wherein the substrate enters a second brush box. In thisoperation, the chemical solution may be dripped onto the top box or thehardware of the scrubber 30 is used to spray the chemical solution ontothe backside of the substrate to reduce the copper contamination on theback side of the substrate. The wet substrate is then advanced to thespin/rinse/dry module 330 where the wafer is wet and is rinsed withdeionized water is introduced while it rotates at 2,000 rpm until thewafer is dry.

The chemical solution is added to the substrate at a rate ofapproximately 300 milliliters/minute. While in scrubber 90, thesubstrate is scrubbed with at least one or more soft poly vinyl alcoholbrushes 380. The brush rotates generally at 100 revolutions per minute(rpm). Waste water exits from the scrubber at a flow rate ofapproximately 2 gallons per minute.

FIGS. 12A and 12B are a flow chart of one embodiment of the inventionused to remove at least one or more particles from a substrate. Atoperation 400, the substrate is advanced to a position such that thesubstrate is adjacent to a polisher. The polisher performs a firstpolishing operation wherein a portion of the metal layer such as copperis removed. At operation 410, a slurry is introduced over a substrate.The slurry includes an abrasive material such as silica or alumina. Atoperation 420, the polisher uses the slurry to perform a first polish ofthe surface of the substrate in order to remove a portion of ametallized layer comprising a metal such as copper on the substrate. Atoperation 430, the surface of the substrate is polished in a secondpolishing operation using a polisher to remove a portion of the metallayer comprising a metal such as tantalum. At operation 440, thesubstrate is rinsed with a solution comprising an acid and preferably achelating agent. A weak organic acid is preferable such as acetic acid,citric acid, gluconic acid, glucoronic acid, oxalic acid, and tartaricacid. At least one particle is removed from the surface of a substrateusing a scrubber and the solution containing an acid such as a weakorganic acid at operation 450. The chelating agent is added to thesolution in order to prevent metals such as copper from adsorbing to thesurface of the substrate. At operation 460, the substrate is advanced toa spin/rinse/dry module. At operation 470, the substrate is dried inless than two minutes at a temperature approximately in the range of 20°C. to 25° C. Other temperatures may be used.

In the preceding detailed description, the invention is described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the claims. The specification and drawings are, accordingly, tobe regarded in an illustrative rather than a restrictive sense.

1. A metal layer cleaning composition comprising: a first component thatis an acid; and a different second component that is a chelating agent,wherein the composition is in a form suitable for use in effectivelyremoving undesirable constituents from a wafer surface following apolishing operation.
 2. The composition of claim 1, wherein the acid isselected from the group consisting of acetic acid, citric acid, gluconicacid, glucuronic acid, oxalic acid, and tartaric acid.
 3. Thecomposition of claim 1, wherein the chelating agent is selected from thegroup consisting of ethylenediaminetetraacitic acid, ethylenediamine,nitrilotriacetic acid, glycin, diethylene triamine, and triethanolamine.
 4. The composition of claim 1, wherein the pH of the compositionis acidic.